Colour Bar: the Triumph of Seretse Khama and his Nation by Susan Willliams 432pp, Penguin, £25

At the heart of this engaging book is the story of an enduring love affair between a black man and a white woman, which began one summer night in gloomy, rationed postwar Britain. But this was no ordinary man, nor indeed any ordinary woman. He was Seretse Khama, the heir to the kingship of the largest tribe of an African protectorate under British control; she was Ruth Williams, a 23-year-old clerk in a shipping company, and a conservative, with a small and large c.

As Susan Williams shows in this extensively researched and elegantly written account, the love story of Seretse and Ruth defines an era of dying colonial power. Stymied in their relationship at every turn by the British government, in covert alliance with apartheid South Africa, the dignity of Khama and his strong-willed bride came to represent the emerging freedoms and racial tolerance of Africa as a whole.

The young Khama was sent over to London in 1945 to study law by his uncle, the Regent of the Bangwato tribe to which Seretse was heir. Lonely at first in the chill world of Oxford, he moved to London, where he met several other politically minded young Africans; and then Ruth, at a dinner dance, in June 1947. Within months, the couple were engaged.

Almost immediately, the young mixed-race couple faced trouble. They were plagued by racist landlords and casual abuse in the streets. British government officials, family friends and church figures tried to prevent the marriage. Four days after their first attempt to wed in a Kensington Church was blocked by the Bishop of London, Seretse and Ruth were married in a civil ceremony. The bride wore a black suit.

In late 1947, Seretse returned to Bechuanaland to seek ratification of his marriage from his tribe: at an extraordinary tribe assembly, thousands of men stood up in a dramatic show of support for their future Chief. Sadly, the British response was not so sophisticated: under intense pressure from South Africa, which bordered Bechuanaland, and in alliance with Khama's uncle, who violently opposed the marriage, they began to find ways to block the return of Khama to his native country.

Williams has done a masterly job in unravelling and chronicling a shameful piece of colonial history, in particular, the twists and turns of the Harragin special inquiry and its political aftermath. Set up to decide whether Seretse was a fit and proper person to discharge the functions of Chief, the inquiry found in his favour but nonetheless argued that South Africa's opposition to his marriage, and therefore his chieftainship, constituted enough reason to bar Khama from returning to his country.

This "inflammable document" caused Attlee great political difficulty. As he wrote in January 1950, "it is as if we had been obliged to agree to Edward VIII's abdication so as not to annoy the Irish Free State and the United States of America." Khama was summoned to Britain, and was discourteously made to wait through a general election before being informed of his banishment from his homeland and the postponement of any decision about the chieftainship for five years.

Khama fought a dignified campaign against his painful seven-year exile in Britain. His case became a cause celebre among MPs, left and right, who kept up insistent pressure in Parliament, as well as prominent actors, journalists and churchmen. In early 1952 the then Tory government, hoping to keep Khama as far from Africa as possible, insulted him with the offer of an administrative post in Jamaica. He refused.

The government's eventual capitulation was in large part due the advancing tide of colonial freedom. In late 1956, Alec Douglas Home, the Commonwealth Secretary, persuaded Eden, preoccupied with the impending Suez debacle, of the case for negotiating a return home on condition that he renounce his claims to be chief.

Susan Williams has succeeded in the difficult feat of seamlessly entwining a political and personal story. She conveys the human aspects of her tale, from the pettiness of the white settler population to the distinctive personalities of Ruth and Seretse - she, fiery; he, charmingly even-tempered - just as powerfully as the political ins and outs of this famous case.

The even-handed narrative also cleverly underlines the casual racism, arrogant patronage and incredible hauteur of both Labour and Tory politicians. Writing of his 1950 meeting with the Commonwealth Secretary, Patrick Gordon Walker, in which he was first informed of his banishment, Seretse Khama was most struck by the politicians' manner: "as unfeeling as if he was asking me to give up smoking, or surrender old school (examination) papers that I had accumulated while at Oxford. I doubt that any man has been asked to give up his birthright in such cold, calculating tones."

Ultimately, though, this is a story of love and redemption. After his return home, Seretse Khama was duly elected first democratic head of the newly created nation state of Botswana, which he ruled for over 20 years before his death from cancer in 1980. Ruth, who adapted remarkably easily to life both in Africa and the political spotlight, took her place as the mother of the nation during Seretse's life and after.

As for Khama himself, he never let anger sour his outlook. "I myself," he said on a 1967 visit to Malawi, "have never been very bitter at all. Bitterness does not pay. Certain things have happened to all of us in the past and it is for us to forget those and look to the future. It is not for our own benefit, but for the benefit of our children and children's children that we ourselves should put this world right."

· Melissa Benn and Clyde Chitty's A Tribute to Caroline Benn: Education and Democracy is published by Continuum